The present invention relates to pressure gauges and in particular to an improved McLeod gauge suitable for use with mercury or other fluids.
McLeod gauges are well known in the art and come in a variety of different forms including the swivel type, the piston type and the tilting type. See U.S. Pat. No. 3,417,622 for an example of an improved swivel type McLeod gauge; U.S. Pat. No. 3,122,923 for an example of an improved piston-type McLeod gauge and Vacuum Technology (November 1962), pages 50-52 for an example of an improved tilting type McLeod gauge.
Commercial McLeod gauges generally employ mercury as the fluid used to compress the gas--i.e., the pressure measuring fluid. While such gauges have been found to be generally satisfactory in use, there exist particular applications for which they are unsuitable due to the possibility of mercury vapor escaping from the gauge into the system whose pressure is to be measured--e.g., biological systems wherein mercury would act as a poison, high vacuum systems wherein the mercury vapors would interfere with ionization gauges or condense on gold or silver surfaces and cause dangerous amalgamation, etc.
McLeod gauges employing a fluid other than mercury have been used in research work. Typically such gauges utilize in place of the mercury a low density, non-volatile fluid, such as silicone oil. Not only do these gauges avoid the danger arising out of the presence of mercury vapor, but because of the lower density of the silicone oil relative to mercury, the sensitivity of the gauges is increased by a factor of almost 15. Furthermore the silicone oil is far cheaper than mercury, an importent consideration in light of the high volume of fluid required. On the other hand, the silicone oil McLeod gauges have not been successfully used on a commercial basis because liquids like silicone oil have a relatively high gas absorption compared to mercury. Thus gas from the liquid evolves in the closed end of the gauge during storage and gives anomalous readings unless highly complex apparatus is used to degas the liquid prior to use. Thus there remains a need for a McLeod gauge capable of utilizing a liquid other than mercury and yet not requiring complex degassing apparatus.
Accordingly, it is an object of the present invention to provide a McLeod gauge suitable for use with either mercury or low density nonvolatile fluids, such as silicone oil.
Another object is to provide such a gauge in which the fluid may be degassed by a simple process adequate to remove all evolved gases and yet not requiring complex degassing apparatus.
A further object is to provide such a gauge which is less expensive to manufacture than a mercury McLeod gauge.
It is yet another object of the present invention to provide such a gauge capable of utilizing a low density, non-volatile fluid and thus affording a greater sensitivity than a similar gauge utilizing mercury.